Hi Avalon,
I've been working on making an argument using (quasi) formal logic. While that's not complete yet, I think I've reached a point that illuminates a crucial issue.
Here's the start I've made.
Basic notation:
^ and
v or
~ not (negation)
-> implies
where n stands for any statement,
I

means "I choose n"
G

means "God [the omniscient agent] knows or predicts n"
P

means "n is possible"
Where n stands for any true statement (theorem),
Omniscience premises:
Omniscience
n -> G

G

-> n
It also follows (as contrapositives) that
~G

-> ~n
~n -> ~G
Possibility premises:
Actuality Implies Possibility
n -> P
Impossibility
~P

-> ~n (contrapositive from the previous premise)
Free will premises:
There exists A such that:
P(I(A)) ^ P(~I(A)) (free will)
and
~(I(A) ^ ~I(A)) (necessary choice)
(Example of A: "I eat of at least one apple between noon and 1:00 PM EDT on June 1, 2012")
Now, I've found I can't get anywhere without an additional premise we might call "divine consistency":
~(G

^ G(~n))
Without this, we get theorems applying to a world in which the omniscience is achieved by knowing or predicting every possibility. This might describe merely an inadequate definition of omniscience, or a (perhaps branching) universe in which every possibility comes to pass.
Now, to prove a contradiction in these premises, we need to show something like
G

-> ~P(~n)
which would then lead to a contradiction with the free will premise. However, as I've defined "possibility" so far, with a single weak premise stating that what occurs must be possible, it appears this can't be done.
It seems, intuitively, that there must be a more powerful notion of possibility than my current one, as my current one is wholly subordinate to actuality.
On the other hand, it does not appear that you would accept G

-> ~P(~n) as a premise; that indeed, that nonacceptance is your very point of contention in this thread.
In English, that disputed premise is: If an omniscient being predicts n, then it is necessarily impossible for not-n to occur.
So let me ask you: If an omniscient being predicts n, then in what sense is not-n possible?
Respectfully,
Myriad